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Enhanced IDE, also called EIDE, is a term that Western Digital coined in
1994 to represent a particular set of extensions it devised to the
original AT Attachment standard. At that time, the official ATA standard was rather
limiting, and work was progressing towards the new ATA-2 standard.
Western Digital decided that it did not want to wait for the new standard, and also that
it could better position itself as a market leader by creating a new feature set for
(then) future drives. The name "Enhanced IDE" was presumably selected to build
upon the common name for ATA then in popular use: IDE.

The original Enhanced IDE program included the following improvements over ATA:

ATA-2 Enhancements: EIDE includes all (most?) of the improvements that are
defined as part of the ATA-2 standard, including the
higher-speed transfer modes.

ATAPI: The EIDE definition includes support for non-hard-disk ATAPI devices on the IDE/ATA channel. Note that at
that time, ATAPI was not part of the ATA standard at all.

Dual IDE/ATA Host Adapters: The EIDE standard specifically includes support for
dual IDE/ATA channels, allowing four IDE/ATA/ATAPI devices to be used. (In fact, the ATA
standard at the time never precluded the use of two IDE/ATA channels; it just was not
commonly done.)

EIDE has become a widely-accepted term in the industry, which would be great if not for
the fact that it is so incredibly confusing. Objections to EIDE include the following
issues:

Proprietary Standard: EIDE is not an official
standard, and it competed with other non-standard IDE/ATA terms like Fast ATA. Of course, that criticism applies not just to EIDE.

Scope: Much of the criticism of the original EIDE program is that its
scope was too wide, and that it encompassed features that are really the domain of the
BIOS. For example, support for dual IDE/ATA host adapters, meaning a secondary IDE/ATA
channel, has nothing to do with the interface or the hard disk itself. And ATAPI is a
standard that is defined for use with optical drives and other non-hard-disk devices,
which again requires BIOS and driver support and really has nothing to do with the hard
disk. At the time, other hard disk manufacturers not only excluded these from their own
standard proposals (such as Fast ATA), they made a point of
criticizing Western Digital for bringing these issues into the interface discussion.

The Word "Enhanced": The choice of the word
"enhanced" was unfortunate, as it led to confusion in another area. At around
the same time that EIDE was introduced, the 504 MB
hard disk size barrier became a big issue. To work around this required an
"enhanced BIOS". Because of the fact that both of these phrases use the word
"enhanced", and because EIDE defines BIOS support standards, many people have
come to think of the terms as interchangeable when they really are not. This has lead to
claims that you need an enhanced IDE interface to support disks over 504 MB, when you
don't--you just need an enhanced BIOS. As if this
weren't bad enough, some companies advertised add-in cards with enhanced BIOSes as
"enhanced IDE cards"! :^)

Redefinition: Since EIDE is Western Digital's term, they have the right
to change its meaning, and unfortunately, they do this on a regular basis. At first, EIDE
included only PIO modes up to mode 3; then mode 4 was added. When the new Ultra DMA modes came out, WD of course added support for them to
their newest models, but they kept calling the drives "EIDE"! Today other drive
manufacturers also say things like "EIDE compatible", leaving you wondering what
exactly this means.

Some people in the hard disk industry apparently feel that the creation of
"Enhanced IDE" was one of the worst things to ever happen to the IDE/ATA
interface! I think that is probably a bit over-stated, though I do agree that it
is probably one of the most confusing things to ever happen to the IDE/ATA
interface. :^) Much of the criticism is valid, but some of it is just the usual conflicts
between rivals in a very competitive industry. And I do think Western Digital's goal of
expanding IDE/ATA capabilities was a laudable one, even if the implementation of the
program left a bit to be desired.

Of all the criticisms leveled at Western Digital, there's one that I personally agree
with strongly, and that's the issue of redefining the term. Every time the IDE/ATA
interface standards change, Western Digital changes the actual interface specifics of its
drives, but continues to list the interface of the drive as just "EIDE". A term
that is constantly redefined is a term that is utterly meaningless. As a result, I can
only tell people at this point that if they see a drive labeled as being "EIDE",
to keep digging to find out the specifics of the modes and official standards it supports,
because "EIDE" by itself doesn't tell you anything (other than the generic
interface of the drive, as the terms "IDE" or "ATA" do.) It would be
nice if Western Digital would just drop the term entirely, but I doubt this will happen
since they have spent so many years promoting it.